When the Amazon Flowed Backwards

The Incredible Story of the Pebas System and How It Shaped Life in South America

In the 1860s, an American paleontologist was studying some unusual fossil specimens found near the town of Pebas in the Peruvian Amazon. The fossils were from a rich bed of mollusc remains, but they didn’t seem that old and included some animals that typically preferred saltier water. This was strange, since Pebas is located over 2,000 kilometers up the Amazon River, far from the ocean.

What the paleontologist didn’t know was that, just 13 million years ago, this area wasn’t the upland jungle it is today. Instead, it was a massive series of lakes and wetlands known as the Pebas System – a sprawling, diverse ecosystem that was fed by a river flowing in the opposite direction of the modern Amazon.

The story of how the Amazon watershed flipped and reshaped the evolution of life in South America is a dramatic tale of tectonic forces, river piracy, and the rise of the Andes mountains. Let’s dive in and explore this incredible chapter of Earth’s history.

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The Eocene Savanna

When South America first started breaking away from Africa around 55-34 million years ago, the highlands under eastern Brazil were actually the uplands of the continent. This meant that during the Eocene epoch, the mighty rivers of South America ran westwards, not eastwards towards the Atlantic.

In fact, if you traveled to Peru during this time, the landscape would have looked quite different from the Amazon rainforest we know today. While parts of the continent to the east were covered in lush rainforest, the western coast was more like a savanna, with forests growing along rivers between scrubby trees and open grasslands.

This was because the Atlantic Ocean was much narrower back then, meaning there was less moisture being blown across South America. Without the towering Andes mountains as a barrier, much of that moisture was just carried out over the Pacific instead of being forced to fall on land.

The animals that inhabited this region were also quite different from what we see today. Fossils found along riverbeds near the town of Santa Rosa in Peru paint a picture of a landscape populated by small, arboreal creatures like rodents and marsupials, as well as larger herbivorous mammals called notoungulates grazing in the grasslands.

The Rise of the Andes

As South America continued to push westward, it began colliding with the oceanic plates to its west, slowly causing the Andes mountains to start forming. By around 23 million years ago, the land in Peru and Colombia had risen enough to create a problem – the rivers could no longer flow uphill over the growing mountain range.

This caused the water to become trapped, unable to drain eastwards towards the Atlantic. Instead, it pooled and swamped the continent’s interior, turning much of South America’s heartland into an expansive system of lakes and wetlands known as the Pebas.

At its peak, the Pebas System covered an area the size of modern-day Egypt – over 1 million square kilometers. It was a hot, humid land of palm swamps and rainforests, teeming with an incredible diversity of life.

• The Pebas was home to a wide array of crocodilians, including the gigantic Purussaurus which could reach 10 meters long – longer than any living crocodile.
• Other strange crocs like Mourasuchus may have used a whale-like feeding strategy, gulping water and straining out small fish and arthropods.
• Opossums, carnivorous marsupials, primates, and even rhino-like Astrapotherium roamed the land.
• The waterways were filled with a spectacular diversity of fish, invertebrates, and other aquatic life.

This diversity was driven by the constant isolation and reconnection of populations as the Pebas landscape shifted, creating new barriers and pathways. Techniques like molecular phylogeny have revealed rapid radiations and speciation events across many different groups of organisms.

The Flip of the Amazon

So what ultimately caused the mighty Amazon watershed to reverse direction and start flowing eastward towards the Atlantic? Once again, the answer lies with the Andes mountains.

As the Andes continued to rise, lifting the land higher, more and more sediment washed down from their upper reaches during rainstorms. This gradually filled in the Pebas System, blanketing it in silt. By around 10.5 million years ago, the land had become so tilted that the waters began to run eastwards instead of westwards.

This phenomenon, known as “river capture” or “river piracy”, represents one of the largest such events in Earth’s history. Over time, more and more water that had once flowed towards the Pebas was rerouted to start running down towards the Atlantic, forming the modern Amazon River system.

The impact of this dramatic reversal was significant for the creatures of the Pebas. Many of the giant crocodilians disappeared completely, while other species like the clam-eating crocs eventually went extinct as well, along with their prey. Carnivorous marsupials also struggled as the thick rainforest gave way to a more open canopy.

However, this was not all doom and gloom. Alongside these extinctions, the combining of previously separate watersheds into the new Amazon, Orinoco, and Magdalena river systems also sparked new waves of diversification, especially among fish and monkeys.

“Over time, things would settle down into the flow that we’re more familiar with. But we can still see the effects of the Pebas and those river captures in what lives there today.”

For example, the connection to the Caribbean that brought in creatures like dolphins and manatees means they still patrol the Amazon’s rivers today. And patches of nutrient-rich soil throughout the region may be remnants of the marine nutrients that washed in during the Pebas era.

Overall, the dramatic tectonic and hydrological shifts that reshaped South America set the stage for the incredible biodiversity we see in the Amazon and surrounding regions today. The constant closing and opening of pathways repeatedly isolated and reunited species, driving them to evolve in spectacular ways.

Uncovering the Past

The story of the Pebas System and the reversal of the Amazon watershed is a remarkable example of how the geological and paleontological record can reveal the dynamic history of our planet. But how do scientists actually uncover these ancient landscapes and the creatures that inhabited them?

One key tool is molecular phylogeny – using estimated rates of genetic change to figure out how recently two species diverged from each other. This has allowed researchers to identify rapid radiations and diversification events across many different groups of organisms that lived in the Pebas.

Fossils also play a crucial role, though the Cenozoic fossil record of the Amazon region is relatively poor, making it challenging to piece together the full picture. Nonetheless, important discoveries like the giant crocodilian skeletons found in Peru have provided invaluable insights.

In 1977, a Chilean farmer plowing his field high in the Andes stumbled upon the fossilized skeleton of a marine reptile.

The Pebas System and the reversal of the Amazon are just one example of how the geological history of our planet has shaped the evolution of life. By unraveling these ancient stories, we gain a deeper understanding of the dynamic processes that have created the world we see today.

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